554 
ME. W. HUGGINS ON THE SPECTEA 
trum of the umbra. The line nearly central between the two lines may be due, in part 
at least, also to sodium*. These lines appeared slightly broader, as if by the addition 
of a faint and narrowed nebulosity at both edges. 
The group of lines at B are stronger, also b and E. Many of the lines marked in 
Kirchhoff’s map as coincident with iron appeared much stronger in the spectrum of 
the spot. 
The absence of sensible increase in F was marked in comparison with a line or lines 
which were very strong, situated at a little distance on the less refrangible side of F — 
probably those of 2066*2 and 2067*1 of Kirchhoff’s map. 
It may be well to consider some of the conditions of the solar surface by which the 
phenomena observed may have been produced. A cooler state of the heated vapours by 
which the lines of absorption are produced, would diminish the radiation from the gas 
itself, and so leave more completely uncompensated the absorption by. the gas of the 
light from behind it. This cause would produce increased blackness of the lines, but 
would not account for more than a slight apparent increase of breadth. The greater 
breadth of the lines seems to point rather to a condition of the gases in which their power 
of absorption embraces for each line an increased range of wave-length. That the power 
of absorption of gases varies in this respect is shown by the increase of breadth which 
some of the bright lines of some gases assume under altered conditions of tension and 
temperature. It will be sufficient to refer to the expansion of the lines of hydrogen as 
the tension increases. A similar increase in the range of its power of absorption on light 
passing through it should take place under similar conditions of density and tempe- 
rature. 
The phenomena may point to an increase of density in the vapours existing within 
the umbra. Such a state of things would necessarily exist at a point somewhat nearer 
the sun’s centre ; but we do not know through how great a depth of gas below the pho- 
tosphere we receive the light which comes from the umbra. Our views on this point 
will be connected with the interpretation we give to Mr. Dawes’s discovery of the exist- 
ence within the umbrae of spots of a still darker part almost wholly devoid of light. 
Does this nucleus represent a more complete unveiling of the inner part of the sun 1 or 
does it show a still cooler and less luminous part of the down-rushing solar atmosphere l 
The latter suggestion, which is in accordance with the explanation of sun-spots proposed 
by Dr. Balfour Stewart, would seem to connect a lower temperature with the broader 
lines of absorption. 
Some information might be gained if we could view the spectra of the dark pores of 
the solar surface, an observation which is perhaps not impossible, since these pores are 
of varying degrees of darkness, and are probably due to conditions of the solar substance 
* The spectrum of the sodium at a high temperature is much more complex than was supposed. In addition 
to the three double lines besides D and a nebulous band, described in my paper “ On the Chemical Elements,” 
Philosophical Transactions, 1864, p. 147, there is a line nearly central between the lines D. It is possible there 
may be also other lines in the interval between the strong lines D x and D 2 . 
